Attention is recently focused on the plate-making system in which a printing plate is subjected to direct plate-making based on digital data sent from a computer or the like without using a lith film, etc. In recent years, remarkable progress has been made in lasers. In particular, with respect to solid lasers and semiconductor lasers which emit light in from near infrared to infrared regions, compact lasers having a high output have become easily available. These compact lasers are extremely useful as the exposure source of a system for direct plate-making from digital data.
In JP-A-7-285275 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") is proposed an image recording material on which an image can be recorded through exposure with an infrared laser. This recording material is a positive image recording material comprising a binder such as a cresol resin, a substance which absorbs light to generate heat, and a substance which is thermally decomposable and, in its undecomposed state, serves to substantially reduce the solubility of the binder, such as quinonediazide. In this positive image recording material, the substance which generates heat upon light absorption emits heat in exposed areas to thereby make the exposed areas soluble.
In JP-A-7-20625 and Japanese Patent Application No. 10-22404 is described a negative image recording material comprising a compound which decomposes by the action of light or heat to generate an acid, a crosslinking agent which crosslinks with an acid, at least one alkali-soluble resin, and an infrared absorber. In this negative image-recording material, the substance which generates heat upon light absorption emits heat in exposed areas and this heat causes the acid-generating compound to decompose and generate an acid, which in turn accelerates the crosslinking reaction between the crosslinking agent and the alkali-soluble resin. Thus, image recording is conducted.
The image recording materials described above can be used as lithographic printing plate precursors (hereinafter referred to as "plate precursors"). In forming a lithographic printing plate, an image is recorded on a plate precursor by exposure with an infrared laser and the exposed plate precursor is subjected to development optionally after heat treatment.
In general, an automatic developing apparatus is employed for the development of the plate precursors. Widely used automatic developing apparatus is of the type in which a plate precursor image-recorded is conveyed into a developing tank and immersed in a developing solution introduced therein and the photosensitive layer in the nonimage areas of the plate precursor is removed with a rubbing means (e.g., a brush roller) disposed in the developing solution.
when the above-described, positive or negative, image recording material, which is capable of image recording through exposure with an infrared laser, is used in forming, e.g., a lithographic printing plate therefrom, there have been cases where the printing plate obtained gives printed matters having image defects e.g., scumming, and image blank spots. Although various attempts have hence been made to overcome the above problem, for example, by changing development conditions in an automatic developing apparatus, they have failed to eliminate such image defects and image blank spots.
There have also been cases where in an automatic developing apparatus, a deposit adheres to the surface of a conveying roller disposed in the space over the developing solution and the deposit scratches the surfaces of plate precursors. An example of such a deposit is a precipitate formed when photosensitive-layer components which have been dissolved in the developing solution separate out as the conveying roller surface wetted with the developing solution dries. The image recording materials, including the positive and negative types, capable of image recording through exposure with an infrared laser have so poor dissolution resistance that even slightly scratched parts of the photosensitive layer surface are dissolved away to leave defects in image areas.